Creosote accumulates in a chimney flue because the surface is somewhat cooler than hot gas rising up through it, so condensation forms on the surface. The condensation catches some of the fire’s combustion byproducts, such as smoke, tar fog, hydrocarbons, water vapor and unburned wood particles. They stick to the moist surface and build up over time as a black or grayish brown sticky crust, which is highly combustible creosote.

   Although it accumulates in all chimneys that burn wood, some conditions—such as restricted air supply, unseasoned wood, and unusually cool chimney temperatures—can accelerate the process. When creosote builds up to a sufficient thickness and the fire temperature is hot enough, the result is a chimney fire. The five photos below are examples of dangerous creosote accumulation in both masonry and manufactured fireplaces. 

    Chimney fires can be especially dangerous in a masonry fireplace. When a fire starts inside a masonry chimney, it burns at about 2000º F, which is hot enough to melt mortar and cause tile liners to cave in. When tiles fracture and mortar falls away, it provides a opening for fire to reach the flammable wood structural members of the home nearby. This is an extremely dangerous event, and the home can be engulfed in flames quickly.

   Although most people assume that their fireplace is made of brick or concrete block, only the older—pre-1960 or so—fireplaces are actually completely made of masonry.  Today’s fireplaces are manufactured from metal in a factory, assembled in the home, and then faced with brick or stone. Most prefabricated fireplaces are rated to withstand temperatures of up to 2100º F without failure but, if a factory-built fireplace does sustain a chimney fire, it should be replaced. 

   Clean chimneys don’t have creosote fires. It’s that simple. Plus, an annual checkup and cleaning by a professional chimney sweep will not only eliminate the fire risk, but includes multiple other safety checks. If you are buying a property, your home inspector will examine and report on the condition of the fireplace hearth and flue, and report on any evidence of creosote buildup.

    Gathering around a crackling fire in the hearth on a winter evening is a satisfying ritual for homeowners lucky enough to have a fireplace, and regular maintenance will keep a creosote fire from spoiling the occasion.

    Also, see our blog post Is a gas log lighter dangerous?

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Here’s links to a collection of our other blog posts about FIREPLACES AND CHIMNEYS:

• What is the 3-2-10 rule for masonry chimneys? 

• What causes black soot buildup on my gas fireplace logs?

• Why does the house have a chimney but no fireplace?

• Why is the chimney leaning away from the house? 

• How is a factory-built fireplace different from a regular fireplace?

• The fireplace doesn't have a chimney. Is that alright? 

    Visit our FIREPLACES AND CHIMNEYS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.